The AlGaInP alloy system has been used for making high quality semiconductor lasers with an emitting wavelength of around 670 nanometers. This alloy system may also be useful for making light emitting diodes (LEDs) for wavelengths ranging from about 560 to 680 nanometers by adjusting the aluminum to gallium ratio in the active region of the device. Increasing the aluminum proportion produces shorter wavelengths. It has also been demonstrated that metalorganic vapor phase epitaxy (MOVPE) provides a means for growing optically efficient AlGaInP heterostructure devices.
There are four types of prior arts of AlGaInP LED pertaining to the present invention. The device geometry of a conventional LED is simple, as shown in FIG. 1, which is disclosed in `Prog. Crystal Growth and Charact`, Vol. 19, 1989, pp. 97-105 by J. P. Andre et al. The LED of FIG. 1 is fabricated with a back electrical contact 110, a substrate of n-type GaAs 120, a double heterostructure of AlGaInP 130, which includes a layer of n-type AlGaInP 131, a layer of undoped AlGaInP 132, and a layer of p-type AlGaInP 133, and a front electrical contact 140. The undoped AlGaInP 132 is technically referred to as an active layer, and the two neighboring n-type AlGaInP layer 131 and p-type AlGaInP layer 133 are referred to as confining layers.
For efficient operation of the LED, current injected by the front electrical contact 140 should be spread evenly in the lateral direction so that the current will cross the p-n junction of the double heterostructure of AlGaInP 130 uniformly to thereby generate light evenly. The p-type AlGaInP layer 133, which is grown by means of the MOVPE process, is very difficult to dope with acceptors of a concentration higher than 1E18 cm.sup.-3. Moreover, it is a material characteristic that hole mobility is low in p-type AlGaInP semiconductors(which is about 10 to 20 cm.sup.2 *V/sec). Due to these two factors, the electrical resistivity of the p-type AlGaInP layer 132 is comparatively high ( i.e., about 0.5 .OMEGA.-cm) so that current spreading is severely restricted. As a result, the current tends to concentrate under the front electrical contact 140. This is often referred to as a current crowding problem.
One technique to solve the current crowding problem is disclosed by Fletcher et al in a U.S. Pat. No. 5,008,718 and in the Journal of Electronic Materials, Vol. 20, No. 12, 1991, pp. 1125-1130. The proposed LED structure is shown in FIG. 2 (in this FIGURE, layers that are not changed in appearance from the structure of FIG. 1 are labelled with the same reference numerals), in which a semiconductor window layer 200 is grown upon the p-type AlGaInP layer 133. The window layer 200 should be selected from materials that have a low electrical resistivity so that current can spread out quickly, and a bandgap higher than that of the AlGaInP layers so that the window layer is transparent to light emitted from the active layer of AlGaInP.
In an LED for generating light in the spectrum from red to orange, an AlGaAs material is selected to form the window layer 200. The AlGaAs material has the advantage of having a lattice constant compatible with that of the underlying GaAs substrate 120. In an LED for generating light in the spectrum from yellow to green, a GaAsP or GaP material is used to form the window layer 200. It is a drawback of using the GaAsP or the GaP material that their lattice constants are not compatible with those of the AlGaInP layers 130 and the GaAs substrate 120. This lattice mismatch causes a high dislocation density that produces less than satisfactory optical performance.
FIG. 3 shows a third prior art LED disclosed in Photonics Spectra, December 1991, pp. 64-66, by H. Kaplan, and in Jpn. J. Appl. Phys. Vol 31 (1992) pp. 2446-2451, by Hideto Sugawara et al. The LED of FIG. 3, in addition to the structure of FIG. 1, is fabricated with a Bragg reflector layer 310, a current-blocking layer 320, and a current spreading layer 330. The current spreading layer 330 has a very low electrical resistivity and the current-blocking layer 320 is arranged at a position where it is in alignment with the front electrical contact 140 and thus is spread out laterally by the current-blocking layer 320. Moreover, the reflector layer 310 can be used to prevent the light emitted by the active layers from being absorbed by the GaAs substrate.
It is a drawback of the LED of FIG. 3 that the fabricating process, in which the MOVPE procedure needs to be performed twice, is very complex.. Moreover, the p-type AlGaInP layer 133 is easily oxidized since it contains a large proportion of aluminum.
To address the drawbacks of the LED shown in FIG. 3, Joji Ishkawa et al. have proposed an indium-tin oxide /InGaAsP/AlGaAs LED structure as shown in FIG. 4. This LED comprises an n-type indium-tin oxide (ITO) window layer 410, an undoped InGaAsP active layer 420, a p-type AlGaAs cladding layer 430, a p-type GaAS substrate 440 and an Au/AuZnNi electrode 450. The ITO window layer 410 provide better current spreading than the AlGaAs window layer under a much lower thickness because it has a lower electrical resistivity. Therefore, current crowding is eliminated and the manufacturing time can be reduced. However, this sort of structure can not be applied to LEDs with n-type GaAs substrates.